The present invention relates to a base station apparatus having a function of detecting the position of a terminal apparatus in communication and its transmit power control method used in a radio communication system.
A method of detecting the position of a terminal apparatus in communication by a base station apparatus in a radio communication system is disclosed in xe2x80x9cRequirements and Objectives for 3G Mobile Services and System (ARIB) 1998.7.21xe2x80x9d, etc.
FIG. 1 is a system diagram showing a radio communication system including a base station apparatus having a position detection function.
When base station apparatus (BS) 1 detects the position of terminal apparatus (MS) 2 carrying out a radio communication, detects direction angle xcex8 of terminal apparatus 2 between the direction of arrival of a reception signal and the base station taking advantage of array antenna characteristics. This method of detecting the direction of the terminal apparatus is disclosed in the xe2x80x9cIntroductory Course for Adaptive Signal Processing Technology Using Array Antenna and High Resolution Arriving Wave Estimationsxe2x80x9d, etc.
After detecting direction angle xcex8 of terminal apparatus 2 formed with the own station, base station apparatus 1 measures the distance between the own station and terminal apparatus 2. The method of measuring distance L between base station apparatus 1 and terminal apparatus 2 will be explained below using a slot timing chart in FIG. 2.
It takes propagation delay xcfx84 for a downlink signal sent from base station apparatus 1 to arrive at terminal apparatus 2. Likewise, it takes propagation delay xcfx84 for an uplink signal sent from terminal apparatus 2 to arrive at base station apparatus 1. Furthermore, it takes apparatus delay xcex4 after terminal apparatus 2 completes reception of the downlink signal until it starts to send the uplink signal. This apparatus delay xcex4 is generated due to delays in processing of the components of the apparatus and timing jitter, etc.
As shown in FIG. 2, slot length S and apparatus delay xcex4 are known to base station apparatus 1, and therefore by measuring time T after starting to transmit the downlink signal to terminal apparatus 2 until base station apparatus 1 starts to receive the uplink signal sent from terminal apparatus 2, it is possible to calculate propagation delay xcfx84 from expression (1) below:
xe2x80x83xcfx84=(Txe2x88x92Sxe2x88x92xcex4)/2xe2x80x83xe2x80x83(1)
Then, base station apparatus 1 can calculate distance L between the own station and terminal apparatus 2 from expression (2) below, where C is the velocity of light:
L=xcfx84xc3x97Cxe2x80x83xe2x80x83(2)
Here, since expression (1) above only applies to a direct wave, it is only a direct wave that is used for position detection.
In contrast, if the system uses a communication system with high resolution with respect to a delay wave such as CDMA system, each apparatus improves its reception quality by carrying out RAKE combination that combines reception signals, which are direct and delay waves arriving at different times. Moreover, each apparatus performs transmit power control to reduce interference in reception processing from other stations while maintaining its desired reception quality.
That is, the conventional base station apparatus always performs transmit power control based on a power value of a signal resulting from RAKE combination of direct waves and delay waves, and so reception power of a direct wave is relatively small, resulting in a problem of failing to detect the position of the target terminal apparatus.
Moreover, if the base station apparatus performs control during position detection so that the terminal apparatus simply increases transmit power, interference in the reception processing of signals by other stations increases, making it impossible to attain the desired reception quality. That is, it is desirable to reduce transmit power of each terminal apparatus to a necessary minimum in consideration of the system as a whole.
It is an object of the present invention to provide a base station apparatus and transmit power control method capable of making reliable position detection compatible with reduction of interference in reception processing of signals from other stations.
The present invention attains the above object by carrying out transmit power control based on reception power of a signal combining direct wave and delay wave signals if position detection is correctly performed, and carrying out transmit power control only based on reception power of the direct wave signal if position detection is not correctly performed.